In connection with microwave communication systems, such as those used in communications satellites, networks are provided for generating antenna beam signals which are used to drive transmit arrays which in turn form transmit beams to transmit communication signals to an intended destination. Early types of beam-forming networks were used in frequency scanning radar to form a frequency scanning beam. This early type of beam-forming network comprised a single periodic delay line in the form of a meandering transmission line and only a single beam was formed at the time. A relatively large scan angle was covered by the beam in a series of small angular steps, each step corresponding to a frequency step in the radar transmitter. Such a system is a time sequential arrangement.
In other applications, resonant circuit delay networks were employed to achieve frequency addressability to antenna beams. Resonant circuits were employed because the frequencies and bandwidths involved were relatively low and transmission lines having the required delays would have been too long to be practical.
The above-discussed beam-forming networks are unsuitable for high frequency communication satellites wherein it is desirable to simultaneously form a plurality of steerable antenna beams to provide downlink communication channels between the satellite and very small earth terminals. An example of such a communications satellite is disclosed in commonly assigned, copending U.S. patent application Ser. No. 896,982 filed Aug. 14, 1986 in the name of H. A. Rosen and entitled SATELLITE COMMUNICATIONS SYSTEM HAVING FREQUENCY ADDRESSABLE HIGH GAIN DOWNLINK BEAMS, which application is hereby incorporated by reference. In the system disclosed in that application, a communications satellite interconnects large numbers of very small aperture earth terminals in a manner which maximizes satellite EIRP as well as the available bandwidth. The system employs highly directional, contiguous beams on the downlink which substantially increases the EIRP and allows multiple reuse of the assigned frequency spectrum. As a result, the number of communication channels that can be provided for point-to-point service is maximized. The high multi-carrier transmitter efficiency is achieved as a result of the dispersion of intermodulation products, and the deleterious effects of rain on the downlink channels are easily overcome by the use of pooled transmitter power. The interconnection of the many users is achieved by a combination of filter interconnection matrix and a highly directional addressable downlink beam.
The beam-forming network used in the communications satellite described in the aforementioned application overcomes each of the deficiencies of the prior art beam-forming networks. This network is described and claimed in commonly assigned, copending U.S. patent application Ser. No. 896,911 filed Aug. 14, 1986 in the name of H. A. Rosen and entitled BEAM-FORMING NETWORK, which application is hereby incorporated by reference.
In brief, this beam-forming network is capable of simultaneously forming a plurality of antenna beam signals for transmission by an antenna to a plurality of zones using a plurality of transmit signals respectively corresponding to the zones, wherein each of the transmit signals includes a plurality of subsignals each destined to be received at an associated location in the corresponding zone. The network includes a first plurality of lines for respectively carrying the plurality of transmit signals and a second plurality of spaced apart lines intersecting the first plurality of lines at crossover points of the two sets of lines. The first and second plurality of lines are coupled with each other at the crossover points by cross guide couplers such that a portion of the energy of each of the transmit signals carried by each of the first plurality of lines is transferred to each of the second plurality of lines whereby the output of each of the second plurality of lines is an antenna beam signal which includes all of the subsignals destined to be received at the associated locations in the corresponding zone. The distance between adjacent crossover points and the width of each of the first plurality of lines are pre-selected to produce a desired shift in the phase of each of the subsignals such that the subsignal are steered to the respectively associated locations in the corresponding zones. Accordingly, the beam-forming network is highly suitable for use in a communications satellite which transmits downlink beams to different locations in various zones, wherein the beams are transmitted to each of the zones over the same range of frequencies to effectively provide reuse of the same range of frequencies in all the zones.
One important advantage of the beam-forming network just described is that it produces time delays in the antenna beam signals without the need for resonant circuits or the like. Another advantage is that the network is especially simple in construction and is easy to manufacture. The network described in the two aforementioned U.S. patent applications is constructed on a single level or plane. Such an arrangement is highly satisfactory when the network is used to drive traditional solid-state power amplifier systems associated with active phased antenna arrays.
Antenna beams formed by an active phase array often require unequal excitation coefficients for the array elements. This is done to achieve either a prescribed side lobe level or to form a prescribed beam shape. Frequency addressable antenna beams transmitted by an active phase antenna array, are required for reasonably efficient operation to have in the scanning direction a narrow beam width, maximum gain, and low side lobe levels. To achieve low side lobe levels, the amplitude distribution of the signal set applied to the transmit array may ideallly resemble a Taylor distribution, which is a symmetrical and tapered distribution of the type described in T. Taylor "Design of Line-Source Antennas for Narrow Beamwidth and Low Side Lobes", IRE Trans. Antennas & Propagation, pp. 16-28 (Jan. 1955).
To achieve such amplitude tapering with a plurality of power amplifiers operated at or very near saturation, and thus at maximum efficiency so as to conserve power, which is typically the most precious resource on a satellite, I conceived and developed a new amplifier system. My new equal power amplifier system is described and claimed in commonly assigned U.S. patent application Ser. No. 032,126, now U.S. Pat. No. 4,825,172, issued Apr. 25, 1989, filed concurrently herewith and entitled EQUAL POWER AMPLIFIER SYSTEM FOR ANTENNA ARRAY AND METHOD OF ARRANGING SAME. My new amplifier system requires an unusual routing of signal lines to the various amplifiers used in the system, which is rather complex to implement when using the beam-forming network described in aforementioned U.S. patent application Ser. No. 896,911.
A principal object of the present invention is reducing the complexity of the signal routing and simplifying the connection of a beam-forming network to the equal power amplifier system described in the just-cited paten application. Another important object of the present invention is to conserve space on the communications shelf of the satellite wherein beam-forming networks are used. Still another object of the present invention is to provide a beam-forming network which, in addition to providing a plurality of antenna beam signals for transmission by an active array antenna to a plurality of zones using a plurality of transmit signals, also provides the amplitude tapering required to produce a predetermined amplitude distribution, such as a Taylor distribution.